The invention relates to radiation sensors and, more particularly, to a Geiger-Mueller triode for detecting the direction of incident ionizing gamma radiation.
It is often desirable to determine the direction or distribution of gamma radiation such as in tomography, astronomy, and civil defense applications, and to locate the sources of radioactivity emitting these photons. Various techniques have been used to provide detectors with a directional capability. One commonly used directional detector is a collimated instrument with shielding restricting the angular acceptance of radiation by the detector and reducing background contributions from other directions. The collimated instruments suffer from several disadvantages including distortion by interactions with the collimator walls, a small solid angle of acceptance thus reducing radiation intensity and poor angular resolution when a large solid angle of acceptance is necessary.
Another approach is described in Byrd et al., U.S. Pat. No. 5,345,084 entitled, xe2x80x9cDirectional fast-neutron detector,xe2x80x9d which discloses a directional radiation detector for detecting fast neutrons, with fast neutron radiation detectors arranged in a close packed relationship to form a segmented symmetric detector. In this device, a processor arithmetically combines the incident radiation counts from the detectors to output a signal functionally related to a direction of a radiation source and the detector only detects the direction of fast neutrons and has a low directional resolution.
Kronenberg et al. U.S. Pat. No. 5,665,970, entitled, xe2x80x9cDirectional Radiation Detector And Imager,xe2x80x9d disclosed forming a radiation sensor by sandwiching two materials having different atomic numbers (Z) around Geiger-Mueller and Scintillator radiation detectors, and solid-state radiation detectors, such as those made of silicon. That patent, which is incorporated herein by reference, described two simple types of GM pancake counters arranged in a back-to-back orientation with a high Z layer of lead located between the counters but external to both GM bodies that would function as a directional sensor. That arrangement provided for detecting photo-Compton electrons and pair electrons emitted from the high or low Z material in the forward or backward directions and the attenuation of the emitted electrons by the high Z material. However, that approach still suffers from the drawbacks of attenuation of electrons caused by the mica windows, lack of commonality sensitivity and the limitations of angular resolution.
Therefore, there is a long-standing need for a Geiger-Mueller type sensor to detect the direction or distribution of directions of incident gamma radiation, and consequently locate the sources of the radioactivity emitting these photons, without suffering from the drawbacks, limitations and shortcomings of other types of Geiger-Mueller (xe2x80x9cGMxe2x80x9d) sensors. This long-standing need has been fulfilled by the GM triode sensor of the present invention comprising a high Z layer of tungsten disposed within a single GM counter to serve as a partition dividing the counter into two subchambers. In this configuration, the tungsten partition provides a source of electrons for each subchamber and insures that the gas composition within each subchamber will be identical, thus achieving a common sensitivity for two GM counters, eliminating attenuation and increasing angular resolution.
In addition to the patents cited above, other pertinent references are:
S. Kronenberg, et al. xe2x80x9cDirectional detector for arrays Of gamma ray and X-ray sources,xe2x80x9d Nuclear Instruments and Methods in Physics Research A vol. 378, pp. 531-540, 1996;
S. Kronenberg, et al. xe2x80x9cLocating and imaging sources of gamma and X-radiation directly or through thick shields,xe2x80x9d Nuclear Instruments and Methods in Physics Research A vol. 387, pp. 401-409, 1997; and
S. Kronenberg et al., xe2x80x9cHigh Angular Resolution Sensing Of Gamma Rays In Space,xe2x80x9d Published in SPIE, The International Society for Optical Engineering, Conference Journal, vol. 3116, pp. 49-56, 1997.
Accordingly, it is an object of this invention to provide a more accurate GM sensor.
It is another object of this invention to provide a GM sensor with tungsten partition to serve as a source of electrons for two subchambers and insure an identical subchamber gas composition, thus achieving common sensitivity for the two GM Counters, far greater accuracy, eliminating attenuation, increasing angular resolution and eliminating signals from background radiation. The important capability of eliminating signals from background radiation is achieved by subtracting output counts of one subchamber from those of the other subchamber, thereby canceling out background.
It is yet another object of the present invention to provide a GM triode sensor having a tungsten partition between each subchamber to detect the direction of incident gamma radiation, and consequently locate the sources of the radioactivity emitting these photons.
These and other objects are accomplished by the GM triode sensor of the present invention, comprising a high Z layer of tungsten disposed within a single housing assembly serving as a partition dividing the counter into two subchambers, or GM counters. The tungsten partition disposed between the subchambers serves as a source of electrons for the subchambers to insure an identical gas composition within each subchamber, thus achieving common sensitivity for the two GM counters, far greater accuracy, eliminating attenuation and increasing angular resolution.
Another embodiment of the present invention is a method for sensing the direction of ionizing gamma radiation, comprising the steps of creating an electrical field in opposing subchambers, separating the subchambers with a partition composed of a first material in a housing composed of a second material, injecting a gaseous mixture within the subchambers, generating secondary electrons in a sidewall from incident gamma radiation, penetrating the outer radiation windows with the secondary electrons, causing ionization charges within the sub-chambers to produce electrical charges and counting the signals for determining a point of origin for the incident gamma radiation.